Frequency sweep for acoustic radiation force impulse

ABSTRACT

Frequency is swept in acoustic radiation force impulse (ARFI) scanning. Different frequencies are used at different times during the ARFI. For example, different frequencies are focused to different depths in the ARFI transmit beam. Since the frequency sweep is used for the ARFI pushing pulse rather than a transmit pulse for which echoes are received, the rate of change of the frequency is not dictated by the speed of sound. The rate of change of the frequency may be adjustable or set based on other factors, such as the type of tissue. In combination with a time varying focal position, the frequency sweep may better compensate for loss as compared to a focus sweep alone. The frequency sweep may better compensate for loss as compared to a single point focus ARFI.

BACKGROUND

The present embodiments relate to acoustic radiation force impulse(ARFI) imaging. By transmitting an ARFI pushing pulse, ultrasound may beused to displace tissue directly or through generation of a shear orlongitudinal wave. The displacement resulting from the pushing pulse maybe measured using further ultrasound scanning. Elasticity, shear, orother types of parametric imaging measure tissue characteristics basedon the displacement caused by the ARFI pulse. Tissue with differentcharacteristics responds to displacement differently.

The ARFI pulse is transmitted as a focused beam. The beam has anhour-glass shape with the narrow portion being at the single focus. Thebeam shape causes a non-uniform response, resulting in lesssignal-to-noise ratio for displacements measured in some locations. As aresult, a limited range of locations are available for measuring tissuecharacteristics for a given ARFI pulse. To measure over a range ofdepths, a rapid sequence of separate ARFI pulses focused at differentdepths is generated. Laterally, the narrow beam width at the focal pointlimits the lateral extent to which measurements may be applied. ARFIpushing pulses are repeated to measure displacement at differentlaterally spaced locations. The repetition of ARFI may cause undesiredtransducer heating and may introduce delays in scanning.

In U.S. Pat. No. 9,332,963, a focus of the ARFI is swept over depth toobtain a more uniform push over depth. As a result, the usable imagingdepth span is extended, and image artifacts caused by the hour glassshape of a fixed focus ARFI pulse are reduced. Due to attenuation, theintensity of the ARFI push pulse decreases with depth. Sweeping thefocus may not be adequate to compensate for this loss.

SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods, instructions, and systems for swept frequency inacoustic radiation force impulse (ARFI) scanning. Different frequenciesare used at different times during the ARFI. For example, differentfrequencies are focused to different depths in the ARFI transmit beam.Since the frequency sweep is used for the ARFI rather than a transmitpulse for which echoes are received, the rate of change of the frequencyis not dictated by the speed of sound. The rate of change of thefrequency may be adjustable or set based on other factors, such as thetype of tissue. In combination with a time varying focal position, thefrequency sweep may better compensate for loss as compared to a focussweep alone. The frequency sweep may better compensate for loss ascompared to a single point focus ARFI.

In a first aspect, a method is provided for swept frequency in acousticradiation force impulse scanning by an ultrasound system. The ultrasoundsystem transmits from an ultrasound transducer a transmit beam with afrequency sweep as an acoustic radiation force impulse. Differentfrequencies of the transmit beam are focused at different depths. Theultrasound system, using the ultrasound transducer, tracks displacementsof tissue at different depths. The displacement is in response to theacoustic radiation force impulse. An image is generated as a function ofthe displacement of the tissue at the different depths.

In a second aspect, a system is provided for swept frequency in acousticradiation force impulse scanning. An ultrasound transducer is fortransmitting an acoustic radiation force impulse in a patient. Atransmit beamformer is configured to generate waveforms for the acousticradiation force impulse. The waveforms resulting in the acousticradiation force impulse have higher frequencies focused to focal zonescloser to a transducer and lower frequencies focused to focal zonesfurther from the transducer. A receive beamformer is configured tooutput data representing spatial locations as a function of receivedacoustic signals responsive to motion of the tissue due to the acousticradiation force impulse. A processor is configured to estimatedisplacement of the tissue in the patient over time as a function of theoutput data. A display is operable to display an image. The image is afunction of the displacement.

In a third aspect, a method is provided for swept frequency in acousticradiation force impulse scanning by an ultrasound system. The ultrasoundsystem transmits from an ultrasound transducer a transmit beam as anacoustic radiation force impulse where different frequencies of thetransmit beam are focused at different depths and have a time varyingfocal position. The ultrasound system tracks, using the ultrasoundtransducer, displacements of tissue at different depths. Thedisplacement is in response to the acoustic radiation force impulse. Animage is generated as a function of the displacement of the tissue atthe different depths.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments and may be later claimedindependently or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a flow chart diagram of one embodiment of a method for sweptfrequency in acoustic radiation force impulse scanning;

FIG. 2 is an example frequency as a function of time for a frequencysweep in ARFI;

FIG. 3 is an example spectrum of a waveform signal at an element forgenerating a transmit beam with the frequency sweep of FIG. 2;

FIG. 4 shows example signal magnitude as a function of time for an ARFItransmit beam with swept frequency and time varying focus;

FIG. 5 shows the spectra of the signals from FIG. 4; and

FIG. 6 is one embodiment of a system for swept frequency in acousticradiation force impulse scanning.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

An ARFI uses a frequency sweep. For example, high frequencies arefocused at shallow depths, and low frequencies are focused at deepdepths. Since acoustic echoes to the ARFI are not received andprocessed, the rate of change of the frequency may be based onconsiderations other than the speed of sound, such as the type of tissue(e.g., different for different attenuation).

In one embodiment, both swept frequency and focus are used for ARFI.Focus and frequency of ARFI push pulse are simultaneously swept. Thetissue of interest, size of the region of interest, and position of theregion of interest may be used to determine the optimal duration of theARFI pulse, rates of change of frequency and focus, and/or rate ofaperture growth.

The frequency sweep may provide for a more uniform push pulse withdepth, limiting signal-to-noise variation and increasing the number oflocations available for measuring tissue characteristics in response toa single ARFI. Transducer heating and delays to transmit multiple ARFIto measure tissue characteristics in a region may be avoided. Using bothfrequency sweep and focus sweep may provide for even greater uniformity,leading to more accurate measurements.

FIG. 1 shows a method for frequency sweep in ARFI scanning by anultrasound system. An ultrasound transmission is used to generate tissuedisplacement. By sweeping the frequency of a given ARFI pulse, a singletransmit beam provides a more uniform distribution of acoustic energy.Different frequencies attenuate differently, so sweeping the frequencyby depth may provide for more uniform distribution for displacingtissue, allowing for greater range of depths and/or for a greaterlateral extent over which displacement may be detected.

The method is implemented by the system of FIG. 6 or a different system.For example, a transmit beamformer is used to generate elementwaveforms, and a transducer generates the ARFI transmit beam in responseto the element waveforms. The transmit beamformer and a receivebeamformer are used to track displacements in tissue caused by the ARFItransmit beam. The ultrasound system generates an image from thedisplacements.

Additional, different, or fewer acts may be provided. For example, themethod is performed without generating an image in act 38. As anotherexample, act 30 is performed without one or more of acts 32 or 34. Inyet another example, acts for estimating tissue characteristics orproperties from the displacements are provided.

The acts are performed in the order described or shown (e.g., top tobottom or numerical), but may be performed in other orders. For example,acts 32 and 34 are performed simultaneously based on the waveformsgenerated for the elements of the transducer.

In act 30, an acoustic radiation force impulse (ARFI) beam istransmitted. The beam has a frequency sweep. An array of elements in anultrasound transducer transmits the ARFI beam converted from electricalwaveforms. The acoustic energy with frequency sweep is transmitted tothe tissue in a patient.

The acoustic waveform is transmitted for generating a shear,longitudinal, or other wave as stress to displace tissue. The excitationis an ultrasound pushing pulse. The acoustic energy is focused to applysufficient energy to cause generation of one or more waves travellingthrough the tissue from the focal location or locations. The acousticwaveform may itself displace the tissue.

The shear wave or waves are generated at the focal region and propagatelaterally, axially, and/or in other directions from the focal region.The waves may travel in multiple directions. The waves reduce inamplitude as the waves travel through the tissue.

To generate the wave, high amplitude or power excitations are desired.For example, the excitation has a mechanical index of close to but notexceeding 1.9 at any of the focal locations and/or in the field of view.To be conservative and account for probe variation, mechanical index of1.7 or other level may be used as the upper limit. Greater (e.g., MIexceeding 1.9) or lesser powers may be used.

The waveforms applied to the elements are generated as continuouswaveforms. The waveforms vary, such as being square wave, sinusoidalwaves, or other unipolar or bipolar alternating waveforms. The waveformdoes not have any extended periods of zero output other than to beginand end the waveform. An extended period is one or more cycles. Theremay be part of each cycle at zero, such as for a unipolar square wave,but another part of the cycle has non-zero (positive or negative) outputper cycle.

The ARFI beam is transmitted with waveforms having any number of cycles.In one embodiment, one, most, or all the waveforms for a transmit eventhave 100-2,000 cycles. The number of cycles is tens, hundreds,thousands, or more for the continuous transmit waveforms applied to theelements of the array for the ARFI beam. Unlike imaging pulses that are1-5 cycles, the ARFI pushing pulse has a greater number of cycles togenerate sufficient stress to cause the wave (e.g., shear wave) fordisplacing tissue with an amplitude sufficient to detect. A pushingpulse beam of acoustic energy is transmitted. ARFI is transmitted fromthe array by application of the continuous transmit waveforms to theelements of the array over the period.

The length of the transmission in combination with the amplitudeprovides acoustic power to the tissue. This power may cause a rise intemperature in the tissue. Transmitting along the same or adjacent scanlines may cause the tissue to increase in temperature over time.Biological effects may include hyperthermia at tissue temperature ofabout 41-45° C., protein denaturation at temperatures above 43-45° C.,and tissue necrosis at temperatures above 50° C. Tissue stiffness may beaffected even at temperatures below 43-45° C. At temperatures above43-45° C., increases in viscosity and/or stiffness may occur. Attemperatures above 50° C., the tissue may have a high stiffness and/orhigh attenuation. Biological effects are limited by preventing atemperature increase of over 2 degrees Celsius. Alternatively, thetransmissions may cause biological effects.

By using a swept frequency with a swept focus, a broader region issubjected to tissue displacement for each ARFI beam, resulting in lesstemperature rise for the transducer and/or the tissue to scan a givenregion of interest. The excitation is focused at a plurality oflocations to allow detecting of the resulting shear wave or waves over abroader range of depths in tissue of interest (e.g., tissue regionsurrounding and/or including a possible tumor). Focusing differentfrequencies at different depths may allow for sampling displacements ina broader region of tissue than using a single focus or single frequencyover a time varying focus.

The ARFI beam is generated with constant amplitude modulation. In analternative embodiment, the amplitude of the transmit beam is variedover time with the frequency change. For example, greater amplitudes areprovided for focal locations at deeper depths. The amplitudes of theelectrical waveforms applied to the elements to generate the beam varyas a function of time over the period. The variation rate is constantbut may vary.

The ARFI beam is generated with a constant aperture. For example, all128 or 256 elements are used for each frequency. In other embodiments,the aperture size (e.g., channel mask) varies with the time varyingfrequency. To maintain constant F#, the size of the aperture mayincrease or decrease based on frequency. Some elements are used for somebut not all frequencies of the sweep. The aperture may be smaller (e.g.,fewer number of elements) for lower frequencies and larger for higherfrequencies.

The ARFI transmit beam has a frequency sweep. During the period oftransmission, the frequency content changes. For example, a centerfrequency for an initial cycle is different than the center frequencyfor a last cycle. In one embodiment, a chirp is used where the frequencychanges linearly with time. FIG. 2 shows an example. In otherembodiments, a non-linear frequency sweep over time is used. Stepped orvariation in slope of the frequency as a function of time may be used.Similarly, the frequency band shifts over time. The range offrequencies, the lowest frequency, and/or the highest frequency variesover time during the continuous ARFI transmission. Any measure for theedge of the band may be used, such as 10 or 20 dB down from a peak.

The same frequency sweep is provided for each of the element waveforms.Each element transmits acoustic energy at a same or similar (e.g.,within 5%) center frequency at the same or different times. Where thefocus is fixed, the relative delay or phasing dictates the frequency foreach element so that the acoustic energy generated by the element has asame or similar frequency when converging at the focal location. Wherethe center frequency for the elements varies over time, the frequency ofthe transmit beam at the focal location or locations also varies overtime. The frequency sweep is implemented by the transmit beamformerchanging the frequency of pulsing by pulsers and/or by generation of thewaveforms having the frequency sweep. For the transmit beam generated inresponse to application of the waveforms to the transducer, thefrequency at the focal location or locations of the transmit beam variesover time.

In combination with a swept focus, the transmit beam of the ARFI hasdifferent frequencies transmitted to different depths. Different centerfrequencies are transmitted to different depths. As the frequencychanges over time in the frequency sweep, the focal depth varies in atime varying focus. Successively higher frequencies are focused tosuccessively closer focal zones, or vice versa. Any range of locationsand/or range of frequencies may be used. The change in focal depth overtime may result in change in the relative frequency between elementwaveforms so that the desired frequency is provided at the desired focallocation.

The focus for generating the wave or waves is swept in act 32. To sweepthe focus, the location of the focus is changed over time. A given ARFItransmission occurs over a period. One or more elements begin outputtingacoustic energy first and others join at different times based onrelative delay or phasing. The waveforms for each element continue, withsome ending before others end. This period is for one beam or pushingpulse transmission and extends from the time at which the array startsgenerating acoustic energy and ends at the time at which the array thenceases generating acoustic energy. The sweep in focus and/or frequencysweep occurs during the period for generating a given transmit beam.

The excitation is focused using a phased array with or without amechanical focus. A mechanical focus may be provided for a givendirection, such as an elevation focus. Azimuth and/or axial mechanicalfocus may be provided. In at least one direction and possibly all three(e.g., axial, azimuth, and elevation), the array of elements iselectronically focused. The electronic focus allows variation of thefocus during the period. The sweep of the frequency occurs in thewaveforms applied to each element.

The time varying focus may have a line focus. The line is straight orcurved. The line is continuous but may be for multiple discrete regions(not continuous). The focus changes position over the period duringwhich the transmit beam is generated.

To sweep the focus, the phase profile, delay profile or both are alteredover the period in act 32. Other approaches for sweeping the focus maybe used. For transmitting a beam with a single focal point, the phaseprofile is constant in time. During the period, the same relative phasebetween the waveforms of the different elements is used. For a sweptfocus, the phase profile across the array varies during the period. Theamount of relative phase difference between two or more waveforms ofelements of the array changes. The phase is changed differently for somewaveforms and corresponding elements than for others. To focus at adifferent location, a different phase profile is used. The phase profileis different by having different relative delays between the elements.In sweeping the focus to move the focal location, the different delayprofiles are implemented. At different times in the period, differentrelative phasing is applied.

As a given waveform for a given element is generated, the phase relativeto other waveforms changes. A phase rotator or delay adjustment may beused. Alternatively, the waveform is generated with the variance inrelative phase or delay. The changes in relative phases or delays acrossthe array cause parts of the respective waveforms to be focused atdifferent locations. The focus for the transmit beam varies during theperiod for which the beam is generated. The waveforms are generatedand/or applied to the transducer to include both the variation to sweepthe focus and the frequency sweep.

Any change function may be used for the phase or delay. In oneembodiment, the phase profiles constantly change over time. The changeoccurs every N clock cycles of the transmit beamformer wherein N is aninteger. N may be 1 for constant change. For less frequent change, N maybe a greater number. In other approaches, constant change is providedwith N greater than one but sufficient to cause less than 2 dB down fromthe peak along a continuous focal region.

Depending on the focal locations, the phase for one or more waveformsand/or elements may not change at all or not change for a number ofclock cycles. For example, the phase applied to the waveform for acenter element of an array where the focus is swept axially along anormal to the array may be the same for the entire period. Phases forthe waveforms of elements at the ends of the aperture may have thegreatest variance and rates of change. For the array, the one or morephase terms are constantly changing.

The phase change may be implemented using a phase rotator. In oneembodiment, the phase is controlled using different phase terms, such asconstant phase, linear phase, and quadratic phase terms. Additional,different, or fewer phase terms may be used. To maintain the samerelative phase for a single focal point, the phase terms are constant intime. To sweep the focus, one, more, or all the phase terms may varyover time. The phase terms for a swept focus are channel and timedependent rather than just being channel dependent for a single focuslocation.

The rate of change in the phase profile or phase for given elements isconstant. The rate may be different for different elements depending onthe steering angle and the element position within the aperture. Inother embodiments, the rate of change in phase varies. For example, therate of change may be slower for some range of focal locations toincrease the dwell time or amount of acoustic energy transmitted whilefocused in that region or range of locations. The rate of change may bezero for some focal locations, at least for a part of the period. Thefocal position may vary over time with discrete steps in position ratherthan constant variance.

A single transmit beam is transmitted with the time varying focalposition and frequency sweep. In other embodiments, a given transmitevent may form more than one beam (e.g., simultaneous multi-beam). One,some, or all the transmit beams have time varying foci and frequencysweep. Whether transmitting a single beam or multiple beams during agiven transmit event, the transmission event occurs over a period ofuninterrupted generation of acoustic energy by the array. At least oneelement is generating acoustic energy at any point during the period.Subsequent beams may be formed in a non-continuous manner, such as byhaving a period of one or more waveform cycles without any of theelements of the array generating acoustic energy. Non-continuous may bebased on most of the elements or other number of elements not generatingacoustic energy.

The focal position and corresponding frequency varies axially, laterallyor axially and laterally. The time varying focal position and frequencyvaries by axial position.

In one embodiment, the frequency sweep is provided by generating theelement waveforms having the frequency sweep and relative delaying orphasing of the waveforms by delays or phase rotators for application tothe transducer. In another embodiment, the element waveforms aregenerated to provide both the frequency sweep and the time varyingfocus. For example, assuming focus and frequency change linearly withtime, then the ARFI pushing pulse is represented as:

s(t)=sin(ω_(max) t+0.5αt ²)   (1)

where t is time, ω_(max) is the maximum angular frequency, and a is arate of change in the angular frequency. For a pulse duration of Δt, therate of change in the frequency is:

$\begin{matrix}{\alpha = {\frac{\omega_{\min} - \omega_{\max}}{\Delta \; t}.}} & (2)\end{matrix}$

Other functions may be used for the rate of change, including linear ornon-linear. A constant or time varying rate may be used. Thetime-dependent focus is represented as:

$\begin{matrix}{{F(t)} = {{{\frac{d_{\max} - d_{\min}}{\Delta \; t}t} + d_{\min}} = {{\beta \; t} + d_{\min}}}} & (3)\end{matrix}$

where d is the depth, and β is a rate of change for the focus. Otherfunctions may be used for the rate of change, including linear ornon-linear. A constant or time varying rate may be used.

The time-dependent delay of each transducer element i at azimuthposition x_(i) on a linear array, where i goes from 1 to L, is given by:

$\begin{matrix}{{\tau ( {t,x_{i}} )} = {\frac{\sqrt{{F(t)}^{2} + x^{2}} - {F(t)}}{c}.}} & (4)\end{matrix}$

s(t) of equation 1 may be rewritten as:

s(nT, x=0)=Σ_(n=0) ^(N−1) sin(ω_(max) nT+0.5α(nT)²))δ(t−nT),   (5)

where n is an index, δ is the Kronecker delta function, and T is aperiod between two delta functions. Other delay or phasing functions maybe used. The delta function introduces the relative delay and/or phasinginto the waveform for each element. The transmitted waveform on eachelement is given by:

s(nT, x=x _(i))=Σ_(n=0) ^(N−1) sin(ω_(max) nT+0.5α(nT)²))δ(t−nT+τ(nT, x_(i)))   (6)

Other functions may be used, such as to account for curvature in thearray. The sine function term provides the frequency sweep, and theKronecker delta function term provides the corresponding time varyingfocus.

The waveform generated for each element in the aperture are square wavesor sinusoidal waves. The generated waveforms are applied to the elementsin synchronization, resulting in generation of the ARFI transmit beamwith frequency and focus sweeps.

The rate of change in the frequency and/or the rate of change of thetime varying focus may be adjustable. Different settings may be used.For example, one setting for the rate is used for one type of tissue andanother setting is used for another type of tissue. The size of theregion of interest (i.e., ARFI scan region) and/or position of theregion of interest may also or alternatively be used to set the rate.The same or different information may be used to set the rate of changein frequency and the rate of change in time varying focus. Attenuationand/or other information may be used.

Where the transmit beam is used to receive responsive echoes, then therate of change corresponds to the speed of sound. In dynamic receivefocusing, the focal position changes with the speed of sound. To alignthe transmit and receive focus, then the transmit focus andcorresponding frequency sweep changes with the speed of sound. SinceARFI is used to cause tissue displacement and not to receive responsiveechoes, the adjustable rates may be different than the speed of sound.For example, the rate of change in the frequency may be 100 m/s, whichis different than the speed of sound, 1,540 m/s, by a factor of 10.Other differences may be used.

Since the rate is adjustable, various linear or non-linear functions maybe used. Equation 2 above is one example. Since the rate is adjustable,the rate may be set based on input from the user. For example, the userselects an application. The application indicates the type of tissue.Different tissues have different attenuation and/or absorption as afunction of frequency. The rate of change in the frequency and the focalposition are set based on the selected application. As another example,the position and/or size of the region of interest (e.g., region inwhich shear wave is to be imaged) are input by the user. The range offrequencies and focal positions may relate to the size of the region ofinterest. The position of the region may determine the minimum and/ormaximum frequencies and/or focal positions. The rate of change in thefrequency and/or focal position are set based on the region of interestsize and position. In yet another example, the user directly selects asetting for the rate, such as selecting a rate value. The user inputrate value is used instead of equation 2 or is used to then set themaximum and minimum frequencies to provide the selected rate.

FIGS. 4 and 5 show example transmit beams generated from elementwaveforms based on equation 6. The ARFI pulse duration Δt=400 μs. Thefrequency changes from f_(max)=4 MHz to f_(min)=2 MHz. The initial focusis d_(min)=2 cm and final focus d_(max)=10 cm. As a result, the rate ofchange in the frequency

${\alpha = {\frac{\omega_{\min} - \omega_{\max}}{\Delta \; t} = {{- {\pi 10}^{10}}\mspace{14mu} ( {1\text{/}s^{2}} )}}},$

and the rate of change in focal depth

$\beta = {\frac{d_{\max} - d_{\min}}{\Delta \; t} = {200\mspace{14mu} {( {m\text{/}s} ).}}}$

FIG. 4 shows the ARFI signal of the transmit beam at different depths(e.g., 3, 5, 7, and 9 cm) as a function of time. The transmit beam isgenerated using frequency sweep and time varying focus. Differentfrequencies are transmitted to different depths. While not possible tovisualize the frequency of oscillation with time due to the longduration of the pulse in FIG. 4, a chirp with maximum energy at thefrequency of interes (e.g., high frequencies at shallow depths and lowfrequencies at deep depths) are provided. FIG. 5 shows the spectra ofthe signals of FIG. 4. In FIG. 5, the spectra show that at a particulardepth, the majority of the push energy is concentrated in a narrowfrequency band. FIG. 4 represents the same information as FIG. 5, but inthe time domain. Due to attenuation and/or diffraction, the amplitude ofthe signals at the different depths is different. Amplitude modulationmay be used to make the amplitudes more similar.

In act 36, the ultrasound system, using the beamformer, transducer, andan image processor, tracks displacements in the tissue caused by thewave generated from the ARFI transmit beam. The beamformer generatestransmit waveforms, the transducer converts the waveforms to transmitbeams and converts echoes of the transmit beams to receive signals, thebeamformer forms receive beams from the receive signals, and the imageprocessor correlates receive beams or data from the receive beams todetermine displacements.

The tissue response is a function of the wave created by the ARFI beamand the tissue characteristics. The displacement of the tissue over timemay be expressed as a convolution of the waveform and the tissuecharacteristics or response. The tissue response reflects viscoelasticproperties of the tissue. To measure the viscoelastic properties, thedisplacement of the tissue over time in response to the pushing pulse ismeasured. The displacement of the tissue caused by the created wave orthe ARFI pulse itself is determined over time. As the wave passes agiven location, the tissue displaces by an amount or distance thatincreases to a peak amount and then decreases as the tissue returns torest.

In act 36, the displacement is calculated as a function of time. Thetissue is scanned multiple times to determine the displacement, such asscanning a region at least ten times to determine displacements at ninedifferent times. The tissue is scanned using any imaging modalitycapable of scanning for displacement during the tissue's response to thepushing waveform. The scan occurs over a range of times where thedesired waveform (e.g., shear wave) would be passing through the tissue.

For ultrasound scanning, the wave is detected at locations adjacent toand/or spaced from the focal region for the ARFI pushing pulse. Todetect tissue response to waves in a region of interest, transmissionsare made to the region, and detection is performed in the region. Theseother transmissions are for detecting the waves or displacement ratherthan causing the wave or displacement. The transmissions for detectionmay have lower power and/or short pulses (e.g., 1-5 carrier cycles) anduse the same or different scan line as the ARFI beam. The transmissionsfor detection may have a wider beam profile along at least onedimension, such as laterally, for simultaneously forming receive samplesalong a plurality of scan lines.

The ARFI transmit beam is not used for receiving echoes. The frequencyof the tracking waveforms used for transmit and the frequency forreceive are independent of the frequencies used for the ARFI beam. Forexample, the ARFI has a 1-3 MHz chirp while the tracking beams areB-mode beams with a center transmit frequency of 1.5 MHz and a receivefrequency of 1.5 MHz or 3 MHz harmonic. Since the ARFI is not used forreceiving, the signal from the ARFI does not or has limited interferencewith the receive signals for tracking.

The wave or displacement may be monitored in one, two, or moredirections. A region of interest is monitored to detect the wave. Theregion of interest is any size. Since the focal region is extended usingthe swept focal positions and/or frequency sweep, the monitoring may beover a greater depth, lateral range, area, or volume. Waves aregenerated along the focus. For the axially extended focus in oneexample, an area of 4 cm in depth and 6 mm in azimuth may be monitored.Laterally spaced locations are monitored for each depth independently.The displacements are tracked at each of a plurality of laterally spacedlocations for each depth. The tracking is performed without combiningthe information over a range of depths. This is possible sincesufficient intensity of pushing is applied over the axial extent,allowing for ARFI imaging with greater depth resolution.

The detection region is monitored by ultrasound. The monitoring isperformed for any number of scan lines. For example, four, eight, ormore receive beams are formed in response to each monitoringtransmission. After transmitting the frequency swept ARFI excitation togenerate the wave or displacement, B-mode transmissions are performedrepetitively along one or more transmit scan lines and receptions areperformed along corresponding receive scan lines. In other embodiments,only a single receive beam or other numbers of receive beams are formedin response to each transmission. Some of the ultrasound data, such asat the beginning or end of the repetitions, may not be responsive to thewave or displacement.

An image processor calculates the displacements from the ultrasound scandata (e.g., beamformed samples or B-mode detected data). The tissuemoves between two scans. The data of one scan is translated in one, two,or three dimensions relative to the data in the other scan. For eachpossible relative position, an amount of similarity is calculated fordata around a location. The amount of similarity is determined withcorrelation, such as a cross-correlation. A minimum sum of absolutedifferences or other function may be used. The spatial offset with thehighest or sufficient correlation indicates the amount and direction ofdisplacement for a given location. In other embodiments, a phase offsetof data received from different times is calculated. The phase offsetindicates the amount of displacement. In yet other embodiments, datarepresenting a line (e.g., axial) at different times is correlated todetermine a shift for each of a plurality of depths along the line.

Displacements are determined for a given location at different times,such associated with sequential scans. The displacement is determinedwith respect to an initial or reference frame of scan data (i.e.,cumulative displacement). Alternatively, the displacement is determinedfrom the immediately prior frame of scan data, such assigning theprevious frame as the reference on an ongoing basis (i.e., incrementaldisplacement). The temporal profile for a given location indicatesdisplacement caused by the wave over time.

Where the focal region extends sufficiently for the desiredmeasurements, a single ARFI pushing pulse is used. Where a broaderregion of interest exists despite the time-varying focus, the acts 30-36may be repeated. The transmissions and receptions for displacementdetection are interleaved with ARFI beams to scan different regions oftissue. To monitor a larger region, acts 30-36 are repeated for otherlocations. For each receive beam location, a time profile of motioninformation (i.e., displacements) is provided. A separate time profileis provided for each axial depth and/or lateral location.

The displacement information, with or without a time profile, is used todetermine a characteristic of the tissue. The characteristic isdetermined at each location. Any characteristic may be determined, suchas an elasticity, strain, shear velocity, longitudinal wave velocity,modulus, or other viscoelastic property. The displacements themselvesmay be used to represent the tissue, such as the magnitude of thedisplacement.

In act 38, an image is generated. The image represents the tissuecharacteristic or property. The image is a function of thedisplacements. Using the displacements themselves or a characteristicderived from the displacements (e.g., shear modulus or velocity),information to be displayed is calculated. For example, a numerical ortextual indication of the property may be displayed. In otherembodiments, a plot and/or fit line and slope value are output. Forexample, displacement over time is displayed for each of one or morelocations. The viscoelastic property is communicated to the user in theimage. The image may be a graph, such as a plot of values as a functionof location.

The image may include a one, two, or three-dimensional representation ofthe property, displacement, or other wave information as a function ofspace or location. For example, the shear velocity throughout a regionis displayed. Shear velocity values modulate color for pixels in aregion in a gray-scale modulated B-mode image. The image may representdisplacement information, such as shear or moduli (e.g., the shearmoduli) for different locations. The display grid may be different fromthe scan grid and/or grid for which displacements are calculated. Color,brightness, luminance, hue, or other characteristic of pixels ismodulated as a function of the information derived from thedisplacements.

In other embodiments, the displacements are used for shear wave velocityimaging. The distribution of shear velocities in a two orthree-dimensional region are determined and mapped to image values. Inanother embodiment, shear wave velocity point quantification isperformed. The value of the shear wave velocity at a location isdisplayed as text or a numerical value.

In one embodiment, the image is a function of displacements fromdifferent depths. Using one, two, or three-dimensional imaging, thedifferent locations of tissue represented in the image include differentdepths. For numerical or textual information, the displacements fromdifferent depths are used to derive the value or values for differentdepths. Due to the swept focus of the ARFI beam, displacement atdifferent depths may be detected. The displacement for different lateralpositions is detected. By extending the focus for the ARFI beams, morelocations in an area or volume may be monitored and used for imaging. Byusing the frequency sweep, the signal-to-noise ratio for the locationsis better than without the frequency sweep. A larger region may besampled and/or the determined tissue characteristic is more accuratethan without the frequency sweep. The intensity as a function of depthfor the ARFI may be more uniform using the frequency sweep.

FIG. 6 shows one embodiment of a system 10 for frequency sweep in ARFIscanning. Ultrasound generates tissue displacement, such as throughcreation of a shear or longitudinal wave, and scan data responsive tothe tissue responding to the displacement is used to determine aproperty. For increased region size for measuring displacement for agiven ARFI transmit and/or better signal-to-noise ratio for themeasurement locations, the ARFI is transmitted with a frequency sweep.Further improvement is provided by focusing different frequencies todifferent depths.

The system 10 is a medical diagnostic ultrasound imaging system. Inalternative embodiments, the system 10 is a personal computer,workstation, PACS station, or other arrangement at a same location ordistributed over a network for real-time or post acquisition imaging.Data from a beamformer-performed scan is available through a computernetwork or memory for processing by the computer or other processingdevice.

The system 10 implements the method of FIG. 1 or other methods. Thesystem 10 includes a transmit beamformer 12, a transducer 14, a receivebeamformer 16, an image processor 18, a display 20, and a memory 22.Additional, different or fewer components may be provided. For example,a user input is provided for manual or assisted designation of a regionof interest for which information is to be obtained or for entry of anapplication, type of tissue, and/or setting of rates of change infrequency and/or focal location.

The transmit beamformer 12 is an ultrasound transmitter, memory, pulser,waveform generator, analog circuit, digital circuit, or combinationsthereof. The transmit beamformer 12 is configured to generate waveformsfor a plurality of channels with different or relative amplitudes,delays, and/or phasing. The waveforms include frequency variation overtime. The frequency is swept or varied linearly or non-linearly from thestart to the end or during a portion of the continuous waveform. For agiven ARFI (e.g., 100-1000 cycles), the frequency (e.g., centerfrequency and/or band) of the waveform changes. This frequency variationis performed for all or a sub-set of the channels of the transmitbeamformer 12.

In one embodiment, the waveforms are generated and applied to atransducer array with a time varying focus. For example, the relativephasing varies over time during the generation of the transmit beam. Thewaveforms of each channel incorporate the phase variation, resulting inan ARFI pulse or beam with swept focus or multiple focal locations. Incorrespondence with the frequency variation, different frequencies arefocused to different depths or locations. For example, 1 MHz is focusedat 10 cm, 3 MHz is focused at 3 cm, and other center frequencies arelinearly mapped to locations along a line from 3 cm to 10 cm between 3MHz and 1 MHz. Due to differences in attenuation by frequency, higherfrequencies are focused to focal zones closer to a transducer 14 andlower frequencies focused to focal zones further from the transducer 14.Alternatively, higher frequencies may be focused further than lowerfrequencies.

The frequency range used for the sweep is adjustable. The attenuationand/or absorption of the tissue being examined may be used to set thefrequency range. For example, liver tissue has different attenuationthan breast tissue, so different frequencies and resulting range of thesweep are different for the different types of tissue.

The position and size of the region of interest may determine thefrequency range. The size and position determine the locations beingscanned. Due to attenuation, the frequency used for the deepestlocations may be lower than the frequency used for the closestlocations. Higher frequencies are desired for resolution, butattenuation limits the frequencies that propagate to the deeperlocations.

The frequencies used (e.g., the frequency range) are independent of thespeed of sound. Rather than a frequency sweep where the slope of thefrequency over time is based on the speed of sound, the frequency sweepmay be independent of the speed of sound. Where signals are received forechoes from the transmission, the dynamic receive focusing results inthe transmit frequency sweep having frequencies focused at differentdepths having to match the dynamic focusing, which occurs at the speedof sound. Since there is no receive operation performed for the ARFI,the frequencies and corresponding focal locations may change at anyrate. The rates of change of frequency and focal location areadjustable, such as user settable, settable based on application (e.g.,tissue type), and/or setting based on region size and position.

In another embodiment for time varying focus, the transmit beamformer 12include phase rotators 13 in each channel. Each phase rotator 13 iscontrolled to apply a phase at a time to a generated waveform or awaveform being generated. The phase rotators 13 of the channels of thetransmit beamformer 12 are configured to apply different phase profilesacross the aperture of the transducer 14 over time and/or to the sameongoing waveforms. The resulting waveforms are generated by the transmitbeamformer 12 for creating an ARFI pulse. The phase rotators 13 respondto changes in phase as needed to sweep the focus during a singletransmit beam. The focus is shifted laterally, axially, or both, such ascreating a line of focal points over time in a same transmit beam.

In another embodiment, the waveform generators of the transmitbeamformer 12 (e.g., pulsers or memory with digital-to-analogconverters) generate the waveforms for the channels with the relativedelay or phasing as part of the waveform. For example, the Kroneckerdelta function is used as described above for equation 6 to implementthe time varying focus with the frequency sweep.

The transmit beamformer 12 connects with the transducer 14, such asthrough a transmit/receive switch. Upon transmission of acoustic wavesfrom the transducer 14 in response to the generated waveforms, one ormore beams are formed during a given transmit event. At least one beamis an ARFI pulse with a frequency sweep with or without a swept focus.

For scanning tissue displacement, a sequence of other transmit beams aregenerated after the ARFI is transmitted. The sequence of transmit beamsscans a one, two or three-dimensional region. Sector, Vector®, linear,or other scan formats may be used. The same region is scanned multipletimes. The scanning by the transmit beamformer 12 occurs aftertransmission of the ARFI pulse. The same elements of the transducer 14are used for both scanning and displacing tissue, but differentelements, transducers, and/or beamformers may be used.

The transducer 14 is a 1-, 1.25-, 1.5-, 1.75- or 2-dimensional array ofpiezoelectric or capacitive membrane elements. The transducer 14includes a plurality of elements for transducing between acoustic andelectrical energies. For example, the transducer 14 is a one-dimensionalPZT array with about 64-256 elements.

The transducer 14 connects with the transmit beamformer 12 forconverting electrical waveforms into acoustic waveforms and connectswith the receive beamformer 16 for converting acoustic echoes intoelectrical receive signals. The transducer 14 transmits the ARFI. Thetransmit beam of the ARFI is focused at a tissue region or location ofinterest in the patient. The acoustic waveform is generated in responseto applying the electrical waveforms to the transducer elements. TheARFI causes tissue displacement, either directly or through generationof a wave (e.g., shear wave).

For scanning with ultrasound to detect displacement (tracking), thetransducer 14 transmits acoustic energy based on further waveforms fromthe transmit beamformer 12 and receives echoes. Receive signals aregenerated in response to ultrasound energy (echoes) impinging on theelements of the transducer 14.

The receive beamformer 16 includes a plurality of channels withamplifiers, delays, and/or phase rotators, and one or more summers. Eachchannel connects with one or more transducer elements. The receivebeamformer 16 applies relative delays, phases, and/or apodization toform one or more receive beams in response to each transmission fordetection. Dynamic focusing on receive may be provided. The receivebeamformer 16 outputs data representing spatial locations using thereceived acoustic signals. Relative delays and/or phasing and summationof signals from different elements provide beamformation. In alternativeembodiments, the receive beamformer 16 is a processor for generatingsamples using Fourier or other transforms.

The receive beamformer 16 may include a filter, such as a filter forisolating information at a second harmonic or other frequency bandrelative to the transmit frequency band. Such information may morelikely include desired tissue, contrast agent, and/or flow information.In another embodiment, the receive beamformer 16 includes a memory orbuffer and a filter or adder. Two or more receive beams are combined toisolate information at a desired frequency band, such as a secondharmonic, cubic fundamental, or another band.

The receive beamformer 16 outputs beam summed data representing spatiallocations. Data for a single location, locations along a line, locationsfor an area, or locations for a volume are output. The data may be fordifferent purposes. For example, different scans are performed forB-mode or tissue data than for shear wave detection. Alternatively, thescan for B-mode imaging is used for determining tissue displacements.The receive beamformer 16 outputs data representing spatial locations asa function of received acoustic signals responsive to motion of thetissue due to the ARFI. The receive beamformer 16 does not operate whiledirect echoes from the ARFI impinge on the transducer 14, so the receivebeamformer 16 is configured to output the data without acoustic echoesfrom the ARFI.

The processor 18 is a B-mode detector, Doppler detector, pulsed waveDoppler detector, correlation processor, Fourier transform processor,application specific integrated circuit, general processor, controlprocessor, image processor, field programmable gate array, digitalsignal processor, analog circuit, digital circuit, combinations thereofor other now known or later developed device for detecting andprocessing information from beamformed ultrasound samples.

In one embodiment, the processor 18 includes one or more detectors and aseparate processor. The separate processor is a control processor,general processor, digital signal processor, graphics processing unit,application specific integrated circuit, field programmable gate array,network, server, group of processors, data path, combinations thereof orother now known or later developed device for determining displacementand/or calculating tissue properties. The processor 18 is configured bysoftware and/or hardware to perform the acts.

In one embodiment, the processor 18 estimates tissue displacement overtime as a function of the output data from the receive beamformer 16.The displacements are estimated as a profile or data representing acurve of magnitude of displacement as a function of time. Thedisplacement profile may be obtained by correlating or otherwisedetermining level of similarity between reference data and data obtainedto represent the tissue at a different time.

The processor 18 is configured to calculate tissue characteristics formthe displacements of the tissue over time. For example, a shear velocityis calculated from the displacement over time. The amount ofdisplacement divided by the time provides velocity. In one embodiment,the processor 18 calculates viscosity and/or modulus. The processor 18may calculate other properties, such as strain or elasticity. In yetother embodiments, the processor 18 determines the maximum displacementor other characteristic of displacement or the displacement profile asthe characteristic.

The processor 18 generates and outputs image or display values mappedfrom the property to the display 20. For example, the shear modulus orother value is determined. A text or numerical indication of theproperty is displayed to the user. A graph of the property over time maybe displayed.

In one embodiment, the property is displayed as a function of location.Displacements for a limited number of locations are available inresponse to an ARFI pulse with a single focus. With a swept focus forthe ARFI pulse, displacements for a larger number of locations andrespective larger linear, area, or volume extent are available. With afrequency sweep, better signal-to-noise is provided for the locations,allowing a larger region to be used for a single ARFI. Values, graphs,and/or tissue representations may be displayed using the displacementsat different locations. By using the swept focus as compared to a singlefocus for the ARFI pulse, a same number of locations may be monitoredwith fewer ARFI pulse transmissions for quasi-real-time (e.g., 5-19 Hz)imaging. By using the swept focus as compared to a single focus for theARFI pulse, greater spatial resolution for displacements andcorresponding tissue characteristics may be provided. By using thefrequency sweep, a more uniform wave generation may be provided,resulting in better signal-to-noise ratio than using a swept focuswithout a frequency sweep. The estimates of displacement are more likelyaccurate due to the better signal-to-noise ratio, so provide betterinformation for diagnosis, prognosis, and/or treatment to the physician.

For a representation of the tissue, the magnitude of the tissuecharacteristic modulates the color, hue, brightness, and/or otherdisplay characteristic for different pixels representing a tissueregion. The processor 18 determines a pixel value (e.g., RGB) or ascalar value converted to a pixel value. The image is generated as thescalar or pixel values. The image may be output to a video processor,look-up table, color map, or directly to the display 20.

The processor 18 and the transmit beamformer 12 operate pursuant toinstructions stored in the memory 22 or another memory. The instructionsconfigure the processor 18 and/or the transmit beamformer 12 foroperation by being loaded into a controller, by causing loading of atable of values (e.g., phase profile table), and/or by being executed.The transmit beamformer 12 is configured by the instructions to causegeneration of an ARFI beam with a frequency sweep with or without aswept focus. The processor 18 is programmed for measuring tissuedisplacement and generating an image.

The memory 22 is a non-transitory computer readable storage media. Theinstructions for implementing the processes, methods and/or techniquesdiscussed herein are provided on the computer-readable storage media ormemories, such as a cache, buffer, RAM, removable media, hard drive orother computer readable storage media. Computer readable storage mediainclude various types of volatile and nonvolatile storage media. Thefunctions, acts, or tasks illustrated in the figures or described hereinare executed in response to one or more sets of instructions stored inor on computer readable storage media. The functions, acts or tasks areindependent of the particular type of instructions set, storage media,processor or processing strategy and may be performed by software,hardware, integrated circuits, firmware, micro code and the like,operating alone or in combination. Likewise, processing strategies mayinclude multiprocessing, multitasking, parallel processing, and thelike. In one embodiment, the instructions are stored on a removablemedia device for reading by local or remote systems. In otherembodiments, the instructions are stored in a remote location fortransfer through a computer network or over telephone lines. In yetother embodiments, the instructions are stored within a given computer,CPU, GPU or system.

The display 20 is a CRT, LCD, projector, plasma, or other display fordisplaying two-dimensional images or three-dimensional representations.The display 20 displays one or more images representing the tissuecharacteristic or other information derived from displacements. As anexample, a two-dimensional image or three-dimensional representation ofdisplacement or tissue characteristics as a function of location isdisplayed. Alternatively or additionally, the image is a graph, anumber, or text representation of a value or graph. For example, a shearvelocity, shear modulus, strain, elasticity or other value or graph isdisplayed as the image.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

I (we) claim:
 1. A method for swept frequency in acoustic radiationforce impulse scanning by an ultrasound system, the method comprising:transmitting, by the ultrasound system and from an ultrasoundtransducer, a transmit beam with a frequency sweep as an acousticradiation force impulse where different frequencies of the transmit beamare focused at different depths; tracking, by the ultrasound systemusing the ultrasound transducer, displacements of tissue at differentdepths, the displacement being in response to the acoustic radiationforce impulse; and generating an image, the image being a function ofthe displacement of the tissue at the different depths.
 2. The method ofclaim 1 wherein transmitting comprises transmitting the transmit beamwith the frequency sweep and a time varying focal position.
 3. Themethod of claim 1 wherein transmitting comprises generating the transmitbeam with element waveforms that are continuous over time.
 4. The methodof claim 3 wherein transmitting comprises transmitting with the acousticradiation force impulse having one hundred or more cycles.
 5. The methodof claim 1 wherein transmitting comprises transmitting with a change inthe frequencies over time being adjustable.
 6. The method of claim 5further comprising receiving user input, a setting for the change beingbased on the input from the user.
 7. The method of claim 5 whereintransmitting comprises transmitting with the change different than baseda speed of sound, and wherein tracking comprises tracking withoutreceiving echoes from the transmit beam.
 8. The method of claim 5wherein transmitting comprises transmitting with setting of the changebeing a rate of change set based on a type of tissue, a size of a regionof interest, and a position of the region of interest.
 9. The method ofclaim 2 wherein transmitting comprises transmitting with a rate ofchange in the frequencies over time being adjustable and a rate ofchange of the time varying focus being adjustable.
 10. The method ofclaim 2 wherein transmitting comprises transmitting with the timevarying focal position comprises generating element waveforms as afunction of a Kronecker delta function of a depth range and with a sinfunction of a frequency range.
 11. The method of claim 1 whereintransmitting comprises transmitting the transmit beam with an aperturesize that is time varying.
 12. The method of claim 1 wherein trackingcomprises tracking the displacements laterally and independently at eachof the different depths.
 13. The method of claim 1 wherein trackingcomprises tracking the displacements axially at each of the differentdepths and along at least eight scan lines.
 14. The method of claim 1wherein generating comprises generating the image with pixels modulatedas a function of the tracked displacements in a two or three-dimensionalfield.
 15. A system for swept frequency in acoustic radiation forceimpulse scanning, the system comprising: an ultrasound transducer fortransmitting an acoustic radiation force impulse in a patient; atransmit beamformer configured to generate waveforms for the acousticradiation force impulse, the waveforms resulting in the acousticradiation force impulse having higher frequencies focused to focal zonescloser to a transducer and lower frequencies focused to focal zonesfurther from the transducer; a receive beamformer configured to outputdata representing spatial locations as a function of received acousticsignals responsive to motion of the tissue due to the acoustic radiationforce impulse; a processor configured to estimate displacement of thetissue in the patient over time as a function of the output data; and adisplay operable to display an image, the image being a function of thedisplacement.
 16. The system of claim 15 wherein the transmit beamformeris configured to provide the higher and lower frequencies as a functionof a frequency range, the frequency range being a function of anattenuation of the tissue.
 17. The system of claim 15 wherein thereceive beamformer is configured to output the data without acousticechoes from the acoustic radiation force impulse and wherein thetransmit beamformer is configured to provide the higher and lowerfrequencies independent of a speed of sound.
 18. The system of claim 15wherein the transmit beamformer is configured to provide the higher andlower frequencies based on a setting for a rate of change of frequency,to generate the waveforms for a time varying focal position of theacoustic radiation force impulse, the time varying focal position basedon a setting for a rate of change of focus.
 19. A method for sweptfrequency in acoustic radiation force impulse scanning by an ultrasoundsystem, the method comprising: transmitting, by the ultrasound systemand from an ultrasound transducer, a transmit beam as an acousticradiation force impulse where different frequencies of the transmit beamare focused at different depths and having a time varying focalposition; tracking, by the ultrasound system using the ultrasoundtransducer, displacements of tissue at different depths, thedisplacement being in response to the acoustic radiation force impulse;and generating an image, the image being a function of the displacementof the tissue at the different depths.
 20. The method of claim 19wherein transmitting comprises transmitting with an adjustable rate ofchange of focus for the time varying focal position and an adjustablerate of change of frequency for the different frequencies to thedifferent depths.